The north-south asymmetries in velocity dispersions are detected in this work, though the differences between \(Z>0\)  stars and \(Z<0\)  stars are less pronounced than those for the mean velocities. It is interesting to find that the \(\sigma _Z/\sigma _R\)  in the south disk is larger than that in the north. The velocity dispersions are probes for the secular heating process in the disk, which are principally generated by spirals and giant molecular clouds (GMCs). The variation in \(\sigma _Z/\sigma _R\)  is a measure of the variation of the importance of the GMC as heating agents relative to spirals, since spirals heat mainly within the plane while GMCs heat both in-plane and vertically ([1]}; [2]}; [3]}). The increasing \(\sigma _Z/\sigma _R\)  with increasing \(|Z|\)  indicates that the relative strength of spiral to GMC heating decreases with the distance to the plane within \(|Z|\lesssim 1.5\,\mathrm {kpc}\) . Meanwhile, the larger \(\sigma _Z/\sigma _R\)  below the plane than above suggests either a weaker spiral heating agent or a larger number of GMCs at \(Z<0\) . Moreover, the interactions between a satellite galaxy and the disk could also heat the disk and cause north-south differences in velocity dispersions [3]}.
